What is thirst? It isn't about physical need, but rather a kind of yearning or desire. You can be thirsty for water and ask to drink from the well in your garden, but you can also be thirsty for something intangible like justice. Thirsting is not just about what one wants—it's about what one lacks yet aspires toward.
There are different kinds of thirst. There is the literal kind, when you need water to survive or something to drink because your body is parched from being in the desert. This kind of thirsting might bring someone to ask for water. But there's also a figurative sense of being 'thirsty,' which is about wanting something that can't be consumed.